Here is how to take screenshots in Windows Phone 8.1 without needing any external app :
1. Navigate to the phone screen which needs to have a screenshot taken (e.g home screen).
Press and hold the Volume Up key + Power key together. [For Windows Phone 8, it would be Power key + Start key together]
3. There will be a shutter click which means that the screenshot was taken. It will now be available in Photos > Screenshots album.
Share them or simply let them sync with OneDrive if needed.
Enjoy!